This is another one of those “personal preference” questions.
You can typically mount your tank in the following places :
– the front drawbar if there is room
– rear bumper
– inside a boot secured to a wall
– inside or outside of a tool box.
What is important to know is that the cheaper plastic tanks are not UV stable and will deteriorate over time. You can paint then with UV protection or you can use a metal tank.
My suggestion to people is to locate the tank on the same side of the caravan as the fuel filler of the car. That way it’s only a little extra effort to fill the heater tank while filling the car.